In any biological situation, there are, to a greater or lesser extent, attractive forces between molecules, cells and substrata leading to the attachment of molecules and/or cells to the substrata or to each other. Such forces give rise to effects in a wide variety of situations, some beneficial and others undesirable.
The effects may be grouped into three main areas:
1. Natural environment
Slime layers occur on rocks, in lakes, in rivers or the sea, on boats and ships, or on other submerged structures such as off-shore oil rigs. Apart from the direct effects of such layers, such as increasing the drag resistance on ships in the water, the biological fouling gives rise to other problems, such as the corrosion of surfaces beneath the layer.
2. Medical situations
The formation of dental plaque on teeth is a familiar effect and it will be obvious that there must be attachment of tissue cells to surfaces such as bone.
Likewise, the attachment of platelets from the blood stream at the site of an injury is part of the normal healing process, but the adhesion of blood cells to vessel walls or to each other leads to thrombus formation and the attendant problems.
The inflammatory response in injured tissues involves the attachment of leucocytes to the walls of the blood vessels and the passage of the leucocytes through the walls, giving rise to inflammation in the adjoining tissues.
In addition, it has been shown that normal and diseased tissue cells differ in their adhesive behaviour, while at the molecular biochemical level, the binding effects in the anitgen-antibody reactions (including the uses of monoclonal antibodies) are of particular interest in the study and understanding of immunity and the immune response.
3. Industrial situations
The attachment of cells to surfaces gives rise to costly problems in industry in the fouling of heat exchangers, cooling towers and other process equipment, as well as in pipe-lines where the increased friction due to the biological layer reduces the flow capacity of the pipe. In fermenters and biochemical reactors, growth on walls gives rise to cleaning and other problems, while if probes and sensors become covered, control of a system becomes complicated or breaks down.
Conversely, surface attachment is used as a method of immobilisation of cells in reactors, in which the trickling filter used in waste water treatment is probably the best known example, whilst the surface immobilization of animal cells enters into processes such as the manufacture of vaccines.
As might be expected, the ubiquity of the interactions between cells, molecules and substrata means that there is a common ground between situations which are, apparently, very varied. Generally, the primary attachment is a physico-chemical process, followed by a secondary biological stabilization. However, this basic mechanism of little specificity is complemented by highly specialised adhesion recognition mechanisms, making individual situations very specific. The need for a specific key to open a given lock is a useful analogy.